The discovery of single-molecule and single-nanoparticle surface-enhanced Raman scattering (SERS) has attracted considerable interest, both for fundamental studies of enhancement mechanisms and for potential applications in ultrasensitive optical detection and spectroscopy. A number of researchers have shown that the enhancement factors are as large as 1014-1015, leading to Raman scattering cross sections that are comparable to or even larger than those of fluorescent organic dyes. This enormous enhancement allows spectroscopic detection and identification of single molecules located on the surface of single nanoparticles or at the junction of two particles at room temperature. Progress has been made concerning both the structural and mechanistic aspects of single-molecule SERS, but it is still unclear how this large enhancement effect might be exploited for applications in analytical chemistry, molecular biology, or medical diagnostics. One major problem is the intrinsic interfacial nature of SERS, which requires the molecules to adsorb on roughened metal surfaces. For biological molecules such as peptides, proteins, and nucleic acids, surface-enhanced Raman data are especially difficult to obtain, hard to interpret, and nearly impossible to reproduce. Therefore, a need in the industry exists to improve SERS data for biological molecules.
The current state-of-the-art for viral diagnostic methods involves isolation and cultivation of viruses and may employ (1) an enzyme-linked immunosorbant assay (ELISA), a method that uses antibodies linked to an enzyme whose activity can be used for quantitative determination of the antigen with which it reacts, or (2) polymerase chain reaction (PCR), a method of amplifying fragments of genetic material so that they can be detected. These diagnostic methods are cumbersome, time-consuming, and ELISA has limited sensitivity.
For example, for RSV, isolation of the virus in cell culture has been considered the reference diagnostic method, followed by immunofluorescence assay (IFA) or enzyme immunosorbant assay (EIA). However, results from virus isolation studies are not rapidly available for patient management, and are not sufficiently sensitive to detect infection in a substantial portion of patients. There is, therefore, a critical need for a rapid, reproducible and highly sensitive and specific method of diagnosing viruses such as RSV that inflict substantial disease burdens on human and animal health and (not insignificantly) for other respiratory viruses that also pose a significant threat as agents for bioterrorism. The emergence of biosensing strategies that leverage nanotechnology for direct, rapid, and increased sensitivity in detection of viruses, are needed to bridge the gap between the unacceptably low sensitivity levels of current bioassays and the burgeoning need for more rapid and sensitive detection of infectious agents.
Various bacterial strains are responsible for numerous human diseases. Bacterial infection, such as Escherichia coli, is the cause of millions of hospitalizations and thousands of deaths each year. Current detection and diagnostic methods for many bacterial pathogens are not sensitive enough for early and rapid detection. For instance, the infectious dose of a pathogen such as E. coli O157:H7 is as low as 10 cells and the existing standard for E. coli in water is 4-cells/100 ml, which is far beyond the current detection limit with short detection time. Thus, improved systems and methods for the detection of pathogens and other biomolecules is needed.